Standing up, this bra fits perfectly. It's only when I sit down that the cups ride up and there's a very large gap between the bra and my breasts. It's not very noticeable under thicker tops (which is how I had the bra for so long without noticing - sweaters!) but under a t-shirt it causes the fabric to sit on top of the bra and away from my chest. I tried the 30E on before buying the 30F and the cups were simply too small, so for me sizing down would not be a solution.

If I could stand up all day and never sit, I would say this is the best bra I've ever owned. Unfortunately, I'm more of a couch potato.